1. Field of the Invention
The present invention relates generally to a method of forming very reactive, elemental metal layers by Atomic layer deposition (ALD), more specifically a hydrogen reduction method by plasma enhanced ALD (PEALD), and a high vacuum ALD system configured for realization of this method.
2. Description of the Related Art
ALD is a cyclic process carried out by dividing a conventional chemical vapor deposition (CVD) process into an iterated sequence of self-saturating deposition cycles. See e.g., T. Suntola, Thin Solid Films, 216(1992) 84-89. Unlike CVD where the reacting gases are mixed in the process chamber and continuously react to form a film, ALD reacting gases are delivered separately to react with the surface instead of with each other. Each reaction is self-terminating, depositing a single layer at a time, independent of gas flow distribution or gas transport into substrate features and forming super conformal, continuous coatings in relatively low process temperatures. ALD has been able to provide a critical need for an important technology at a time when no other methods could meet the need. For example, non-planar devices impose geometrical challenges for materials integration, The required conformal deposition necessitates layer-by-layer fabrication that is only delivered by ALD. PEALD is an energy-enhanced ALD method, the surface is exposed to the species generated by plasma during the reactant step. Typical plasmas used during PEALD are those generated in O2, N2, NH3 and H2 reactant gases or combinations thereof. Such plasmas can replace ligand-exchange reactions typical of H2O, and they can be employed to deposit metal oxides, metal nitrides and metal films. PEALD offers several merits for the deposition of ultra-thin films over thermal ALD and other vapor phase deposition techniques such as the high reactivity of the plasma species on the deposition surface, allowing lower deposition temperatures and more freedom for processing on a wider range of temperature—sensitive materials.
A diversity of materials have been deposited successful by ALD technique such as oxides, nitrides, fluorides, sulfides, noble metals and some transition metals with established applications in high κ oxides and transistors, DRAM, magnetic-write heads, and some emerging applications such as gas permeation environmental barriers, and passivation layers for Si solar cells.
One of key challenges currently limiting progress in ALD and critical developments that are needed to advance the ALD state-of-the art is depositing very reactive elemental metal layer. The very reactive metals are defined as highly electropositive elements (Electronegativity χ<1.8) including alkaline metals (Li, Na, K, etc), alkaline earth metals (Be, Mg, Ca, etc), some transition and rare earth metals. Very reactive metal layers are needed for applications such as improved adhesion, barrier, and device performance, etc. . . . However it has been proved very difficult to deposit by either conventional thermal ALD or PEALD in a conventional roughing pump backed reactor where a base pressure is at level of 1 mTorr.
For one reason, these very reactive elemental metals thermodynamically favor in forming much more stable compounds such as fluorides, oxides, nitrides, and carbides, thus it is very hard to avoid contamination by impurity gases such as oxygen, H2O, nitrogen, carbon oxides, etc. in a conventional ALD reactor, or preserve metallic surface after deposition without passivation. These compounds basically act as thermodynamic sinks to prevent formation of pure metals. Aluminum for example is a very reactive metal. Its many=compounds like Al2O3, AlN, and AlC have much lower formation energy than that of the pure aluminum. Mg is another highly electropositive metal. Its compounds like MgF2, MgO/Mg (OH)2, MgC are thermodynamically much more stable than Mg too. In the both cases, impurity levels in the ALD reactor such as chlorine, O2, N2, CO2, and H2O have to be sufficiently low. On the contrary, compounds of many less electropositive metals are less stable thus it is easier to from pure noble metals such as Pd, Ir, Pt, Cu, Ag, and some transition metals such as W, Fe, Co, and Ni by an ALD process.
Another reason is due to lack of precursors and co-reactants for a ALD process, commercially available precursors tend to use similar less electropositive metal precursors which are good for oxides and some for nitrides but create problem with very reactive metals.
The third reason is lack of reducing agents to break up chemical bonds of organometallic precursors for very reactive metals and surrounding ligands. Probably the most suitable reducing agent for any deposition process would be atomic hydrogen. Its advantages are high reducing power, reactivity and chemical compatibility with most processes. The main limitation for using atomic hydrogen is that it needs to be produced in-situ by dissociating molecular hydrogen into atomic ones with a plasma source or a hot tungsten filament. The mean free path for atomic hydrogen is the shortest.
U.S. Pat. No. 8,133,555 described a method for forming metal films by ALD using β-diketone metal complexes and a mixture gases of hydrogen and nitrogen activated by plasma. The applied metal elements cover some transition metals Ti, Mn, Fe, Co, Ni, Cu, Nb, Ta, Hf, Mo, W, and noble metals Rh, Pd, Ir, Pt, Ru, Ag, Au, and alkaline earth Mg, Sr, Ba. US patent application No. 2006/0093848 provide a ALD process of forming noble metals (Ru, Rb, Rd, Ir, Pt) using reducing gases from hydrogen, glyoxylic acid, oxalic acid, formaldehyde, 2-propanol, imidazole and plasma—activated hydrogen. US patent application No. 2008/0102205 invents an ALD process of forming metallic films including elemental metals using metal containing cyclopentadienyl precursors and hydrogen or hydrogen plasma. The metal containing cyclopentadienyl precursors comprises a metal selected from the group consisting of Al, Ga, In, Sc, Ti, V, Cr, Mn, Fe, CO, Ni, Y, Zr, Nb, Mo, Tc, Ru, Rh, La, Hf, Ta, W, Re, Os, and Ir, more preferably from the group consisting of Ti, Zr, Hf, Ta, W, Nb, and Mo. For some materials, direct reduction is difficult, indirect reduction method has to be used such as Fe, Co, and Ni. Ni (Cp)2 cannot be reduced to Ni by atomic H directly, thus first be oxidized to NiO then be reduced to Ni element by atomic hydrogen.